1. Field of the Invention
The present invention relates to a scale for a reflective photoelectric encoder and a reflective photoelectric encoder. The present invention especially relates to a scale for a reflective photoelectric encoder in which a reflective film is not formed on the side faces of phase gratings (hereinafter simply called gratings), and which can stably obtain high diffraction efficiency and allow variations in the shape and size of the gratings caused by processing, even if there are processing variations in the shape and size of the gratings. The present invention also relates to a reflective photoelectric encoder using such a scale.
2. Description of the Related Art
In a scale for a reflective photoelectric encoder, as shown in FIG. 1, many gratings 12 are arranged at predetermined intervals (pitch) p in parallel in a direction orthogonal to the surface of a paper in a longitudinal direction (a lateral direction of the drawing) of a substrate 10.
Diffraction efficiency being an index of signal intensity of a photoelectric encoder using such a scale depends on the shape and size of the gratings (grating width l, grating height h, and edge angle θ). However, variations occur in the shape and size of the gratings (variations in the grating width Δl=approximately 40 nm, variations in the grating height Δh=approximately 10 nm, and variations in the edge angle Δθ=approximately 5 degrees) in processing the scale. Especially in a long scale, it is difficult to uniformly process the shape and size of the gratings over a wide area. There are cases where variations in the shape and size of the gratings cause variations in the diffraction efficiency. At that time, it is difficult to obtain stable high diffraction efficiency.
To solve such a problem, as shown in FIG. 2, Japanese Patent Laid-Open Publication No. Hei 10-318793 discloses that an edge angle θ should be set at 70 degrees±10 degrees, when a scale is combined with a p-polarized light source of λ=670 μm. In the scale, a conductive metal film, especially being a reflective film 14 made of chromium, is formed on the whole surface of gratings 12 of l=256 to 384 nm and h=160 to 210 nm made of a dielectric material such as silicon dioxide, titanium dioxide, tantalum pentoxide, aluminum oxide, or the like.
However, a scale in which a reflective film was not formed on the side faces of gratings 12, as shown in FIG. 3 contrasted with FIG. 2, could not necessarily obtain stable diffraction efficiency depending on a particular width or shape of gratings.
Especially in Japanese Patent Laid-Open Publication No. Hei 10-318793, the gratings 12 are made of silicon with (110). Thus, if the gratings are formed by general anisotropic wet etching, an edge angle is approximately 70 degrees. However, there was a problem that high diffraction efficiency could not be obtained with such an angle.